The C8 alkylbenzenes, ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are often present together in a typical industrial C8 aromatic product stream from a chemical plant or a refinery. For instance, commercially available PxMax, Mobil Selective Toluene Disproportionation and Mobil Toluene Disproportionation processes may produce such a stream.
Of the three xylene isomers, PX has the largest commercial market. PX is used primarily for manufacturing purified terephthalic acid (PTA) and terephthalate esters such as dimethyl terephthalate (DMT), which are used for making various polymers such as poly(ethylene terephthalate), or PET, poly(propylene terephthalate), or PPT, and poly(butene terephthalate), or PBT. Different grades of PET are used for many different popular consumer goods such as films, synthetic fibers, and plastic bottles for soft drinks. PPT and PBT may be used for making similar products with different properties.
Fractional distillation is a commonly used method for many processes in many industrial plants to separate chemicals. However, it is often difficult to use such a conventional fractional distillation technology to separate the EB and different xylene isomers efficiently and economically because the boiling points of the four C8 aromatics fall within a very narrow 8° C. range, from about 136° C. to about 144° C. (see Table 1). The boiling points of PX and EB are about 2° C. apart. The boiling points of PX and MX are only about 1° C. apart. As a result, large equipment, significant energy consumption, and/or substantial recycles would be required to provide effective and satisfactory xylene separations.
TABLE 1C8 compoundBoiling Point (° C.)Freezing Point (° C.)EB136−95PX13813MX139−48OX144−25
Fractional crystallization in a crystallizer takes advantage of the differences between the freezing points and solubilities of the C8 aromatic components at different temperatures. Due to its higher freezing point, PX is usually separated as a solid in such a process while the other components are recovered in a PX-depleted filtrate. High PX purity, a key property needed for satisfactory commercial conversion of PX to PTA and/or DMT in most plants, can be obtained by this type of fractional crystallization. U.S. Pat. No. 4,120,911 provides a description of this method. A crystallizer that may operate in this manner is described in U.S. Pat. No. 3,662,013. Commercially available processes and crystallizers include the crystallization isofining process, the continuous countercurrent crystallization process, the direct contact CO2 crystallizer, and the scraped drum crystallizer. Due to high utility usage and the formation of a eutectic between PX and MX, it is usually more advantageous to use a feed with as high an initial PX concentration as possible when using fractional crystallization to recover PX.
The term “shape-selective catalysis” describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g., by N. Y. Chen, W. E. Garwood and F. G. Dwyer, “Shape Selective Catalysis in Industrial Applications,” 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of a feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational constraints on diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in the dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in selective toluene disproportionation to p-xylene.
The production of PX is typically performed by toluene disproportionation over a catalyst under conversion conditions. Examples include the toluene disproportionation, as described by Pines in “The Chemistry of Catalytic Hydrocarbon Conversions”, Academic Press, N.Y., 1981, p. 72. Such methods typically result in the production of a mixture including PX, OX, and MX. Depending upon the degree of selectivity of the catalyst for PX (para-selectivity) and the reaction conditions, different percentages of PX are obtained. The yield, i.e., the amount of xylene produced as a proportion of the feedstock, is also affected by the catalyst and the reaction conditions.
The equilibrium reaction for the conversion of toluene to xylene and benzene products normally yields about 24% PX, about 54% MX, and about 22% OX among xylenes.
Conventionally, PX production by toluene disproportionation comprises:                a) toluene disproportionation step to produce a product stream having C7− hydrocarbons including benzene and toluene, C8 hydrocarbons including PX, MX, OX, and ethylbenzene, and C9+ hydrocarbons;        b) a separation system comprising:                    1. a C7− separation step to separate the C7− hydrocarbons from the product stream to form a C7− depleted stream; and a C9+ separation step to separate the C9+ hydrocarbons from the C7− depleted stream to form a C7− and C9+ depleted stream which is enriched with C8 hydrocarbons as comparing with the product stream; or            2. a C9+ separation step to separate the C9+ hydrocarbons from the product stream to form a C9+ depleted stream; and a C7− separation step to separate the C7− hydrocarbons from the C9+ depleted stream to form a C7− and C9+ depleted stream which is enriched with C8 hydrocarbons as comparing with the product stream; or            3. a C7− and C9+ separation step to separate C7− and C9+ hydrocarbons from the product stream to form a C7− and C9+ depleted stream which is enriched with C8 hydrocarbons as comparing with the product stream; and                        c) a PX separation step to separate PX from at least a portion of the C7− and C9+ depleted stream.        
Conveniently, the PX separation step (c) normally comprises a crystallization step to produce a PX product with desired purity, e.g., at least 99 wt %. At least a portion of the C7− and C9+ depleted stream is used as a feedstock for the PX separation step (c). Depending on the desired purity of the PX product and depending on the PX concentration in the C7− and C9+ depleted stream, a multi-stage crystallization unit or a multi-stage adsorption unit may be needed.
Crystallization methods can be used to separate PX (p-xylene) from a C8 aromatic starting material which contains ethylbenzene, as well as the three xylene isomers. PX has a freezing point of 13.3° C., MX has a freezing point of −47.9° C. and OX has a freezing point of −25.2° C. However, conventional crystallization methods can be used to make PX with a purity of over 99.5 wt. % only with great expense.
Crystallization processes to recover PX from a mixture of C8 aromatics requires cooling the feed mixture. Because its melting point is much higher than that of the other C8 aromatics, PX is readily separated in the crystallizer after refrigeration of the stream. In conventional PX crystallization processes, the feed contains about 22 to about 23 wt. % PX. This is the type of feed that is generally obtained from catalytic reforming of naphtha, xylene isomerization, and non-shape selective toluene disproportionation (TDP) processes, in which the relative proportion of xylene isomers is close to equilibrium at reaction temperatures. For the production of high purity PX (>99.5 to >99.8 wt %) from these feeds, these feeds are cooled, crystallized and separated at a very cold temperature, normally −65 to −70.5° C. In order to recover most of the PX from solution, the feeds sometimes have to be cooled to as low as about −85° to −95° F. The crystals are melted, and the resulting solution is recrystallized and separated at a warmer temperature for maximum PX purity. Because of the constraint imposed by the eutectic temperature, PX recovery from conventional crystallization processes is generally limited to about 60-65%. Therefore, these processes generally have less favorable economics compared to the newer adsorption based PX recovery technologies, which can recover 97-98% of the feed PX, and have lower capital and operating costs.
U.S. Pat. No. 5,448,005 discloses a crystallization process for PX recovery. A single temperature crystallization production stage is used for producing PX from a feed having a PX concentration above equilibrium, such as from a toluene disproportionation process. Scavenger stages are also used to raise the PX recovery rate.
U.S. Pat. No. 5,498,822 discloses a crystallization process for PX recovery. A single temperature crystallization stage is used for producing PX from a feed having an above equilibrium PX concentration, such as from toluene disproportionation.
Various methods are known in the art for increasing the para-selectivity of zeolite catalysts, for example, U.S. Pat. Nos. 5,349,113, 5,498,814, 5,349,114, 5,476,823, 5,367,099, 5,403,800, 5,365,004, 5,610,112, 5,455,213, 5,516,736, 5,495,059, 5,633,417, 5,659,098, 6,576,582 and 6,777,583.
A modified crystallization process (WO95/26946) may be used when the feed contains a relatively high concentration of PX. The C8 aromatic mixture obtained from selective toluene disproportionation (STDP) processes generally contains over 70 wt % PX. For this type of feed, high recovery of PX is possible using a single production stage at relatively high temperature, −17.8° C. to 10° C. The filtrate is processed through one or more scavenger stages operating at lower temperature, −28.9° C. to −1.1° C., to recover additional PX, which is recycled to the production stage for final purification. When the C8 aromatic mixture contains over 97% PX, it is possible to obtain over 90% recovery in a single production stage operating at −28.9° C. to 10° C., with no scavenger stage (WO95/26947). Such mixtures may be obtained from STDP processes using a silica modified catalyst.
Because of their reduced refrigeration requirements and greater potential recovery of PX, these modified crystallization processes are generally competitive with adsorption based processes. It is believed that the feedstock to the crystallization step (c) requires very low level of C9+ hydrocarbons, which may interfere with the performance of the crystallization unit. Therefore, a C9+ separation step is required to remove C9+ from the product stream of step (a), normally a C9+ distillation column is needed to achieve desired C9+ level in a feedstock for the PX separation step (c).
It has now been surprisingly found that the C9+ separation step may be eliminated or minimized by the combination of high selective toluene disproportionation process which produces a C8 stream and a crystallization process. The elimination or minimization of the C9+ separation step can reduce energy consumption, capital cost, operational cost, and emission to the environment for a PX production plant, which will translate to low PX cost of production and less emission to the local environment.